1. Field of the Invention
The invention relates generally to electrostatic chucks for holding a workpiece and, more specifically, to an improved topographical structure of a support surface of an electrostatic chuck to improve heat transfer gas distribution along the bottom surface of the workpiece.
2. Description of the Background Art
Electrostatic chucks are used for holding a workpiece in various applications ranging from holding a sheet of paper in a computer graphics plotter to holding a semiconductor wafer within a semiconductor wafer process chamber. Although electrostatic chucks vary in design, they all are based on the principal of applying a voltage to one or more electrodes in the chuck so as to induce opposite polarity charges in the workpiece and electrodes, respectively. The electrostatic attractive force between the opposite charges pulls the workpiece against the chuck, thereby retaining the workpiece.
In semiconductor wafer processing equipment, electrostatic chucks are used for clamping wafers to a pedestal during processing. The pedestal may form both a cathode and a heat sink. Electrostatic chucks find use in etching, chemical vapor deposition (CVD), and physical vapor deposition (PVD) applications. More specifically, the electrostatic chuck has a flex circuit covering a support surface of the pedestal. The flex circuit is comprised of a top layer of dielectric material covering a conductive electrode. Below the electrode is a bottom layer of dielectric material. The top and bottom dielectric layers encapsulate the conductive electrode.
In a "unipolar" electrostatic chuck, voltage is applied to the conductive electrode relative to some internal chamber ground reference. Electrostatic force is established between the wafer being clamped and the electrostatic chuck. When the voltage is applied, the wafer is referred back to the same ground reference as the voltage source by a conductive connection to the wafer. Alternatively, a plasma generated proximate the wafer can reference the wafer to ground, although some voltage drop occurs across plasma sheaths that form at both the wafer being clamped and the reference electrode.
The materials and processes used to process a wafer are extremely temperature sensitive. Should these materials be exposed to excessive temperature fluctuations due to poor heat transfer from the wafer during processing, performance of the wafer processing system may be compromised resulting in wafer damage. To optimally transfer heat between the wafer and chuck, a very large electrostatic force is used in an attempt to cause the greatest amount of wafer surface to physically contact the support surface. However, due to surface roughness of both the wafer and the chuck, small interstitial spaces remain between the chuck and wafer that interfere with optimal heat transfer.
To achieve further cooling of the wafer during processing, an inert gas such as Helium is pumped into the interstitial spaces formed between the wafer and the support surface of the chuck. This gas acts as a thermal transfer medium from the wafer to the chuck that has better heat transfer characteristics than the vacuum it replaces. To further enhance the cooling process, the chuck is typically water-cooled via conduits within the pedestal. This cooling technique is known as backside gas cooling.
Since the distribution of Helium to the interstitial spaces is osmotic and the interstitial spaces may not be interconnected, some spaces do not receive any Helium. This condition can lead to a non-uniform temperature profile across the wafer during processing and result in wafer damage. Since effective and uniform heat conduction away from the wafer is an important aspect of the manufacturing process, establishing a uniform Helium layer within the wafer to chuck interface should contribute to uniform heat transfer from the wafer. As such, backside gas cooling art developed based on this premise.
However, the physical limitations of existing technology used to electrostatically clamp large diameter (e.g., 200 mm or more) wafers do not provide the necessary conditions for a uniform distribution of Helium in all of the interstitial spaces beneath the wafer. Existing pedestal topographies limit the effectiveness of the heat transfer process because they have a generally flat top support surface and the wafers have a generally flat bottom surface. Ideally, these flat surfaces would have no defects or deviations so that 100% of the bottom surface of the wafer would contact the top support surface of the pedestal to allow for maximum heat transfer from the wafer to the pedestal. Topographical anomalies create a condition where not all of the wafer is in contact with this support surface. As such, the aforementioned interstitial spaces could form and distribution of the heat transfer gas becomes uneven and uniformity of wafer temperature is compromised. Consequently, during processing, the temperature non-uniformity may result in non-uniform processing and wafer damage.
Therefore, there is a need in the art for an improved topographical structure of an electrostatic chuck that improves temperature uniformity across the wafer without adding considerably to manufacturing costs.